Transcript Transport Layer Support for Highly Available Network Services

Migratory TCP:
Connection Migration for Service
Continuity in the Internet*
Florin Sultan, Kiran Srinivasan, Deepa Iyer, Liviu Iftode
Department of Computer Science
Rutgers University
*Work supported in part by the NSF under grant CCR-0133366
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Today: TCP-based Internet Services
TCP : reliable byte-stream
Client
Server
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The Problem with TCP
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Adverse conditions
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TCP response
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internetwork congestion or failure
server overloaded, failed or under DoS attack
network delays => packet loss => retransmission
TCP limitations

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addressing: implicitly binds the service to a server
error recovery: limited to one server
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Highly Available Server? Not Enough!

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To a TCP client: connect() will succeed most of
the time
To an end-user: may not help beyond connect()

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connectivity failures, server overload, etc.
Service continuity = uninterrupted end-to-end
delivery of a service

as important to end-user as the high availability of
the server!
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Our Solution: Cooperative Service Model
Logically equivalent servers cooperate in sustaining the
service by migration of live connections
Server 1
Client
congestion
Server 2
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Migratory TCP (M-TCP)

A client connection transparently migrates to
different servers during its lifetime

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server applications cooperate to support the handoff
client application does not change
servers can be geographically distributed
Designed and implemented as a TCP extension
Mechanism decoupled from migration policy

policies are not the focus of this talk
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Per-connection State
Server 1
Server 2
Application
Application
Application
state
Protocol
state
M-TCP
M-TCP
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Application - M-TCP Service Contract

Application
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Define fine-grained per-connection application state
Export a per-connection application state snapshot to
the protocol
Import per-connection state after migration and resume
service
M-TCP

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Transfer per-connection state
Synchronize application state with protocol state
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Server Migration API

export_state(conn, state_buff)
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origin server, periodically during service on connection
import_state(conn, state_buff)

destination server, after accepting connection
origin
Application
export()
M-TCP
destination
Application
import()
M-TCP
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Connection Migration Mechanism
Server 1
Client
Server 2
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Prototype Implementation

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Modified TCP/IP stack in the FreeBSD kernel
Real applications
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Icecast audio streaming server

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PostgreSQL transactional DB server front-end
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migrate audio stream
migrate sequence of transactions with ACID semantics
Apache web server
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migrate transfer of static files, dynamic content
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Experimental Setup
10.10.10.10
Control interface
10.10.10.11
S1
S2
Service interface
128.6.171.161
128.6.171.162
Hub
C
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Streaming Server Experiment
Effective throughput close to average rate seen before server
performance degrades
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Related Work

HTTP server fail-over by connection migration
[Snoeren ‘00]
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Fault-tolerant TCP [Alvisi ‘00]
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soft TCP and HTTP state maintained at back-up servers
application-specific: relies on HTTP features
persistent connections across server crashes
failures masked using TCP wrappers
Stream Control Transmission Protocol [RFC 2960]
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multi-homing preserves connectivity if network fails
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Conclusions

Transport layer protocol that supports service
continuity
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dynamic, light-weight, transparent connection migration
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Migration API for server applications
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Working prototype for FreeBSD & real applications
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Future Work
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Multi-process state migration
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application state spans multiple processes that
communicate via IPC channels
Non-intrusive state transfer by remote memory
access (VIA, InfiniBand)
A distribution of the M-TCP software for FreeBSD
will be available this fall
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Thank you!
http://discolab.rutgers.edu/mtcp
Google: Migratory TCP or M-TCP
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